[PATCH 4.14] xen/gntdev: Avoid blocking in unmap_grant_pages()

[Demi Marie Obenour <demi@xxxxxxxxxxxxxxxxxxxxxx>]

commit dbe97cff7dd9f0f75c524afdd55ad46be3d15295 upstream

unmap_grant_pages() currently waits for the pages to no longer be used.
In https://github.com/QubesOS/qubes-issues/issues/7481, this lead to a
deadlock against i915: i915 was waiting for gntdev's MMU notifier to
finish, while gntdev was waiting for i915 to free its pages.  I also
believe this is responsible for various deadlocks I have experienced in
the past.

Avoid these problems by making unmap_grant_pages async.  This requires
making it return void, as any errors will not be available when the
function returns.  Fortunately, the only use of the return value is a
WARN_ON(), which can be replaced by a WARN_ON when the error is
detected.  Additionally, a failed call will not prevent further calls
from being made, but this is harmless.

Because unmap_grant_pages is now async, the grant handle will be sent to
INVALID_GRANT_HANDLE too late to prevent multiple unmaps of the same
handle.  Instead, a separate bool array is allocated for this purpose.
This wastes memory, but stuffing this information in padding bytes is
too fragile.  Furthermore, it is necessary to grab a reference to the
map before making the asynchronous call, and release the reference when
the call returns.

It is also necessary to guard against reentrancy in gntdev_map_put(),
and to handle the case where userspace tries to map a mapping whose
contents have not all been freed yet.

Fixes: 745282256c75 ("xen/gntdev: safely unmap grants in case they are still in 
use")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Demi Marie Obenour <demi@xxxxxxxxxxxxxxxxxxxxxx>
Reviewed-by: Juergen Gross <jgross@xxxxxxxx>
Link: 
https://lore.kernel.org/r/20220622022726.2538-1-demi@xxxxxxxxxxxxxxxxxxxxxx
Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
---
 drivers/xen/gntdev.c | 145 ++++++++++++++++++++++++++++++-------------
 1 file changed, 103 insertions(+), 42 deletions(-)

diff --git a/drivers/xen/gntdev.c b/drivers/xen/gntdev.c
index 7b4ac5505f53..2827015604fb 100644
--- a/drivers/xen/gntdev.c
+++ b/drivers/xen/gntdev.c
@@ -59,6 +59,7 @@ MODULE_PARM_DESC(limit, "Maximum number of grants that may be 
mapped by "
 
 static atomic_t pages_mapped = ATOMIC_INIT(0);
 
+/* True in PV mode, false otherwise */
 static int use_ptemod;
 #define populate_freeable_maps use_ptemod
 
@@ -94,11 +95,16 @@ struct grant_map {
        struct gnttab_unmap_grant_ref *unmap_ops;
        struct gnttab_map_grant_ref   *kmap_ops;
        struct gnttab_unmap_grant_ref *kunmap_ops;
+       bool *being_removed;
        struct page **pages;
        unsigned long pages_vm_start;
+       /* Number of live grants */
+       atomic_t live_grants;
+       /* Needed to avoid allocation in unmap_grant_pages */
+       struct gntab_unmap_queue_data unmap_data;
 };
 
-static int unmap_grant_pages(struct grant_map *map, int offset, int pages);
+static void unmap_grant_pages(struct grant_map *map, int offset, int pages);
 
 /* ------------------------------------------------------------------ */
 
@@ -129,6 +135,7 @@ static void gntdev_free_map(struct grant_map *map)
        kfree(map->unmap_ops);
        kfree(map->kmap_ops);
        kfree(map->kunmap_ops);
+       kfree(map->being_removed);
        kfree(map);
 }
 
@@ -147,12 +154,15 @@ static struct grant_map *gntdev_alloc_map(struct 
gntdev_priv *priv, int count)
        add->kmap_ops  = kcalloc(count, sizeof(add->kmap_ops[0]), GFP_KERNEL);
        add->kunmap_ops = kcalloc(count, sizeof(add->kunmap_ops[0]), 
GFP_KERNEL);
        add->pages     = kcalloc(count, sizeof(add->pages[0]), GFP_KERNEL);
+       add->being_removed =
+               kcalloc(count, sizeof(add->being_removed[0]), GFP_KERNEL);
        if (NULL == add->grants    ||
            NULL == add->map_ops   ||
            NULL == add->unmap_ops ||
            NULL == add->kmap_ops  ||
            NULL == add->kunmap_ops ||
-           NULL == add->pages)
+           NULL == add->pages     ||
+           NULL == add->being_removed)
                goto err;
 
        if (gnttab_alloc_pages(count, add->pages))
@@ -217,6 +227,35 @@ static void gntdev_put_map(struct gntdev_priv *priv, 
struct grant_map *map)
                return;
 
        atomic_sub(map->count, &pages_mapped);
+       if (map->pages && !use_ptemod) {
+               /*
+                * Increment the reference count.  This ensures that the
+                * subsequent call to unmap_grant_pages() will not wind up
+                * re-entering itself.  It *can* wind up calling
+                * gntdev_put_map() recursively, but such calls will be with a
+                * reference count greater than 1, so they will return before
+                * this code is reached.  The recursion depth is thus limited to
+                * 1.  Do NOT use refcount_inc() here, as it will detect that
+                * the reference count is zero and WARN().
+                */
+               refcount_set(&map->users, 1);
+
+               /*
+                * Unmap the grants.  This may or may not be asynchronous, so it
+                * is possible that the reference count is 1 on return, but it
+                * could also be greater than 1.
+                */
+               unmap_grant_pages(map, 0, map->count);
+
+               /* Check if the memory now needs to be freed */
+               if (!refcount_dec_and_test(&map->users))
+                       return;
+
+               /*
+                * All pages have been returned to the hypervisor, so free the
+                * map.
+                */
+       }
 
        if (map->notify.flags & UNMAP_NOTIFY_SEND_EVENT) {
                notify_remote_via_evtchn(map->notify.event);
@@ -274,6 +313,7 @@ static int set_grant_ptes_as_special(pte_t *pte, pgtable_t 
token,
 
 static int map_grant_pages(struct grant_map *map)
 {
+       size_t alloced = 0;
        int i, err = 0;
 
        if (!use_ptemod) {
@@ -322,85 +362,107 @@ static int map_grant_pages(struct grant_map *map)
                        map->pages, map->count);
 
        for (i = 0; i < map->count; i++) {
-               if (map->map_ops[i].status == GNTST_okay)
+               if (map->map_ops[i].status == GNTST_okay) {
                        map->unmap_ops[i].handle = map->map_ops[i].handle;
-               else if (!err)
+                       if (!use_ptemod)
+                               alloced++;
+               } else if (!err)
                        err = -EINVAL;
 
                if (map->flags & GNTMAP_device_map)
                        map->unmap_ops[i].dev_bus_addr = 
map->map_ops[i].dev_bus_addr;
 
                if (use_ptemod) {
-                       if (map->kmap_ops[i].status == GNTST_okay)
+                       if (map->kmap_ops[i].status == GNTST_okay) {
+                               if (map->map_ops[i].status == GNTST_okay)
+                                       alloced++;
                                map->kunmap_ops[i].handle = 
map->kmap_ops[i].handle;
-                       else if (!err)
+                       } else if (!err)
                                err = -EINVAL;
                }
        }
+       atomic_add(alloced, &map->live_grants);
        return err;
 }
 
-static int __unmap_grant_pages(struct grant_map *map, int offset, int pages)
+static void __unmap_grant_pages_done(int result,
+               struct gntab_unmap_queue_data *data)
 {
-       int i, err = 0;
-       struct gntab_unmap_queue_data unmap_data;
+       unsigned int i;
+       struct grant_map *map = data->data;
+       unsigned int offset = data->unmap_ops - map->unmap_ops;
+
+       for (i = 0; i < data->count; i++) {
+               WARN_ON(map->unmap_ops[offset+i].status);
+               pr_debug("unmap handle=%d st=%d\n",
+                       map->unmap_ops[offset+i].handle,
+                       map->unmap_ops[offset+i].status);
+               map->unmap_ops[offset+i].handle = -1;
+       }
+       /*
+        * Decrease the live-grant counter.  This must happen after the loop to
+        * prevent premature reuse of the grants by gnttab_mmap().
+        */
+       atomic_sub(data->count, &map->live_grants);
 
+       /* Release reference taken by unmap_grant_pages */
+       gntdev_put_map(NULL, map);
+}
+
+static void __unmap_grant_pages(struct grant_map *map, int offset, int pages)
+{
        if (map->notify.flags & UNMAP_NOTIFY_CLEAR_BYTE) {
                int pgno = (map->notify.addr >> PAGE_SHIFT);
+
                if (pgno >= offset && pgno < offset + pages) {
                        /* No need for kmap, pages are in lowmem */
                        uint8_t *tmp = 
pfn_to_kaddr(page_to_pfn(map->pages[pgno]));
+
                        tmp[map->notify.addr & (PAGE_SIZE-1)] = 0;
                        map->notify.flags &= ~UNMAP_NOTIFY_CLEAR_BYTE;
                }
        }
 
-       unmap_data.unmap_ops = map->unmap_ops + offset;
-       unmap_data.kunmap_ops = use_ptemod ? map->kunmap_ops + offset : NULL;
-       unmap_data.pages = map->pages + offset;
-       unmap_data.count = pages;
+       map->unmap_data.unmap_ops = map->unmap_ops + offset;
+       map->unmap_data.kunmap_ops = use_ptemod ? map->kunmap_ops + offset : 
NULL;
+       map->unmap_data.pages = map->pages + offset;
+       map->unmap_data.count = pages;
+       map->unmap_data.done = __unmap_grant_pages_done;
+       map->unmap_data.data = map;
+       refcount_inc(&map->users); /* to keep map alive during async call below 
*/
 
-       err = gnttab_unmap_refs_sync(&unmap_data);
-       if (err)
-               return err;
-
-       for (i = 0; i < pages; i++) {
-               if (map->unmap_ops[offset+i].status)
-                       err = -EINVAL;
-               pr_debug("unmap handle=%d st=%d\n",
-                       map->unmap_ops[offset+i].handle,
-                       map->unmap_ops[offset+i].status);
-               map->unmap_ops[offset+i].handle = -1;
-       }
-       return err;
+       gnttab_unmap_refs_async(&map->unmap_data);
 }
 
-static int unmap_grant_pages(struct grant_map *map, int offset, int pages)
+static void unmap_grant_pages(struct grant_map *map, int offset, int pages)
 {
-       int range, err = 0;
+       int range;
+
+       if (atomic_read(&map->live_grants) == 0)
+               return; /* Nothing to do */
 
        pr_debug("unmap %d+%d [%d+%d]\n", map->index, map->count, offset, 
pages);
 
        /* It is possible the requested range will have a "hole" where we
         * already unmapped some of the grants. Only unmap valid ranges.
         */
-       while (pages && !err) {
-               while (pages && map->unmap_ops[offset].handle == -1) {
+       while (pages) {
+               while (pages && map->being_removed[offset]) {
                        offset++;
                        pages--;
                }
                range = 0;
                while (range < pages) {
-                       if (map->unmap_ops[offset+range].handle == -1)
+                       if (map->being_removed[offset + range])
                                break;
+                       map->being_removed[offset + range] = true;
                        range++;
                }
-               err = __unmap_grant_pages(map, offset, range);
+               if (range)
+                       __unmap_grant_pages(map, offset, range);
                offset += range;
                pages -= range;
        }
-
-       return err;
 }
 
 /* ------------------------------------------------------------------ */
@@ -456,7 +518,6 @@ static void unmap_if_in_range(struct grant_map *map,
                              unsigned long start, unsigned long end)
 {
        unsigned long mstart, mend;
-       int err;
 
        if (!map->vma)
                return;
@@ -470,10 +531,9 @@ static void unmap_if_in_range(struct grant_map *map,
                        map->index, map->count,
                        map->vma->vm_start, map->vma->vm_end,
                        start, end, mstart, mend);
-       err = unmap_grant_pages(map,
+       unmap_grant_pages(map,
                                (mstart - map->vma->vm_start) >> PAGE_SHIFT,
                                (mend - mstart) >> PAGE_SHIFT);
-       WARN_ON(err);
 }
 
 static void mn_invl_range_start(struct mmu_notifier *mn,
@@ -498,7 +558,6 @@ static void mn_release(struct mmu_notifier *mn,
 {
        struct gntdev_priv *priv = container_of(mn, struct gntdev_priv, mn);
        struct grant_map *map;
-       int err;
 
        mutex_lock(&priv->lock);
        list_for_each_entry(map, &priv->maps, next) {
@@ -507,8 +566,7 @@ static void mn_release(struct mmu_notifier *mn,
                pr_debug("map %d+%d (%lx %lx)\n",
                                map->index, map->count,
                                map->vma->vm_start, map->vma->vm_end);
-               err = unmap_grant_pages(map, /* offset */ 0, map->count);
-               WARN_ON(err);
+               unmap_grant_pages(map, /* offset */ 0, map->count);
        }
        list_for_each_entry(map, &priv->freeable_maps, next) {
                if (!map->vma)
@@ -516,8 +574,7 @@ static void mn_release(struct mmu_notifier *mn,
                pr_debug("map %d+%d (%lx %lx)\n",
                                map->index, map->count,
                                map->vma->vm_start, map->vma->vm_end);
-               err = unmap_grant_pages(map, /* offset */ 0, map->count);
-               WARN_ON(err);
+               unmap_grant_pages(map, /* offset */ 0, map->count);
        }
        mutex_unlock(&priv->lock);
 }
@@ -1006,6 +1063,10 @@ static int gntdev_mmap(struct file *flip, struct 
vm_area_struct *vma)
                goto unlock_out;
        }
 
+       if (atomic_read(&map->live_grants)) {
+               err = -EAGAIN;
+               goto unlock_out;
+       }
        refcount_inc(&map->users);
 
        vma->vm_ops = &gntdev_vmops;
-- 
Sincerely,
Demi Marie Obenour (she/her/hers)
Invisible Things Lab